The following publications are believed to represent the current state of the art:
U.S. Pat. Nos. 6,199,587; 6,164,587; 5,988,529; 5,947,348; 5,927,608; 5,509,606; 5,383,247; 5,354,965; 5,254,083; 5,118,040; 5,012,977; 4,106,508; 4,090,668 and 3,979,068.
Published PCT Applications: WO 02/092237, WO 00/27540, WO 98/58826.